Linda McQuillan was the heroine known as Captain U.K., the representative of the Captain Britain Corps for Earth-238, as assigned by Merlyn.
When the British government of Earth-238 was taken over by the Omega-Level mutant Mad Jim Jaspers, he outlawed all superhumans and sent his creation, the Fury, to hunt them down. With her husband Rick and she being two of the last surviving heroes of Earth-238, they gathered together with others like themselves and decided to make a stand against the Fury. During said showdown, several of her allies fell: Tom Rosetta, Android Andy, Miracle Man, and Colonel Tusker. Fearing for Linda’s safety, Rick picked her up and carried her to a teleporter booth; with no time to calibrate the destination, he set it to send her as far away as possible. The last sight Linda had of Earth-238 was the Fury grabbing and lifting her husband by the throat and then delivering a killing blast to his chest.
The devastated and terrified Linda ended up on Earth-616. Deciding against making her presence known, she hid out as a photographer. Unbeknownst to her, Brian Braddock of Earth-616 was sent to her home world. There, he faced off with the Fury and lost – ironically enough over the grave that was meant for the absent Captain U.K. While Brian was resurrected by Merlyn, Linda nonetheless sensed his death and shivered.
While still hiding, Linda one day read the newspaper’s headline with dismay; Earth-616’s Jim Jaspers was calling for superhero legislation, exactly what had lead to the deaths of her friends and husband back home. In the mean time, the Fury followed Linda from Earth-238 to her new world. While it was still damaged, it started regenerating itself by using the bodies of homeless people.
Summoning her courage, Linda ventured to Braddock Manor in search of this world’s version of a Captain Britain Corps representative. When she arrived, a group of superhumans were watching a televised speech Jaspers made; this caused her to panic, thinking things were repeating themselves. Fleeing in terror, Linda revealed that she had seen Jaspers deliver the exact same speech on her home world; and that it led to genocide. Fearing the introduction of death camps, Linda started to flee the grounds but was faced with the regenerated Fury on leaving the house itself. Though it shot at her, she was protected thanks to Merlyn’s intervention. Brian and the members of the Special Executive, upon hearing the commotion, came to her rescue – even defeating the Fury temporarily but Linda was rooted by fear.
Jaspers managed to establish his concentration camps for super powered individuals, and left the handling of them to the Vixen who was secretly controlling the government agency S.T.R.I.K.E. (Special Tactical Reserve for International Key Emergencies). A catatonic Linda found shelter in the same basement as other superhumans, most notably Brian, Betsy, and Saturnyne. When Brian set off to face Jaspers, the others were left undefended. In his absence, armored S.T.R.I.K.E. agents – called Beetles – attacked, killing Betsy’s lover Tom Lennox and capturing Betsy and her friend Alison Double. Still terrified, Linda did nothing to help them and instead escaped with Saturnyne. Eventually, Saturnyne bluntly told Linda that she has to help Captain Britain; beating on her when she refused. Finally, Linda snapped and began fighting back out of fear. Pressured by Saturnyne, Linda donned her costume as the world around them began to warp thanks to Jaspers’ powers.
Together, Linda – once more Captain U.K. – and Saturnyne proceeded to 10 Downing Street, where Earth-616’s Jaspers was fighting the Fury. As the Fury didn’t recognize this dimension’s version of Jaspers as its master, it killed him as it would any other superhuman. With Jaspers out of the way, the Fury once more focused on Brian in his persona of Captain Britain. Linda, still caught in her own terror watch as the Fury pinned Brian down and prepared to kill him the same way as it had Linda’s husband, Rick. Filled with rage at seeing things repeating before her, Linda finally overcomes her fear and attacks the Fury, tearing it to pieces and then proceeding to smash said pieces into smithereens. Filled with relief, both captains hold each other, when they are transported to Otherworld by Roma.
On Otherworld, Roma explained that Merlyn had deliberately used Linda as bait to lure the Fury to Earth-616 so that it would kill that Earth’s version of Jaspers. Roma provided Linda and Brian images of the concentration camps being torn down before transporting them back to 616. Linda and Brian share a kiss before separating to go their separate ways.
Sometime later, Linda was approached by Gabriel and Michael, agents of R.C.X. (Resources Control Executive) wanting her help to deal with the Warpies – children who were physically transformed and granted powers by the effects of Jaspers’ reality warp. Linda eventually agreed and helped Brian and the R.C.X..
For a period, Linda was partnered with Betsy Braddock who had temporarily assumed her twin’s mantle of Captain Britain when he stepped down. Linda actively partook in Betsy’s training and worked beside her to fight crime. Deeming her ready to work alone, Linda allowed Betsy her freedom. Unfortunately, very soon after being cut loose, Betsy was brutally attacked and then blinded by the villain Slaymaster. Brian retaliated by killing Slaymaster and then resumed the mantle of Captain Britain.
Some time after that, Linda met with Roma on Darkmoor. Roma told Linda that it was her presence on Earth-616 that was preventing the rift caused by Jaspers from healing completely, and that as such, even more Warpies were being made as a result. Roma granted Linda a gift of seeing what actually happened to her Rick, showing that he was not, in fact, killed by the Fury. Instead, Roma had actually transported Rick away at the last second. Reunited with her husband, they were reassigned to Earth-794, and assigned the task of freeing the world from the despotic rule of the insane Sat-Yr-9, an alternate version of Saturnyne. Soon after being set the task, the couple overthrew Sat-Yr-9, imprisoning her and freeing the Earth.
At a later date, Linda was assigned to Earth-839, where she battled the Red Monarch.
When Brian Braddock was put on trial by the Captain Britain Corps for failing to champion the morality of his Earth (he was shown fighting Nightcrawler because of Brian’s jealousy over Nightcrawler’s feelings towards Meggan), Linda volunteered to act as his defense counsel, even though she risked being found guilty by association. When he realized that the court was prejudiced against him, Brian resisted the court's authority and was attacked by the rest of the Corps, with Linda being his sole ally in the fight. Before events could snowball further, Saturnyne arrived and read a missive by Roma that ended the fight and had the case dismissed.
Linda once again returned to Earth-616 to attend the wedding of Meggan and Brian. During the bachelorette party, Linda and the others were attacked by the Crazy Gang – lead by the new Executioner, Feron, who’d served as a former teammate to Brian and Meggan on Excalibur.
Linda once again joined forces with the married Braddocks when they went to Otherworld for Brian to assume his new role, that of king of Otherworld . They were joined by Psylocke, Black Knight, Sir Benedict, and Crusader X.
Afterwards, Linda reappeared when Brian and Meggan were pulled from Otherworld back into Earth-616 by Brian’s mad older brother, Jamie, as 616 underwent the event reality warp that lead to the event known as the House of M – started by the machinations of an insane Scarlet Witch. Caught up in the delusions of House of M, Linda was again Captain UK and she served as one of the Braddock’s guards, attempting to fight off an onslaught of attacking Sentinels with Justicer Bull, Saturnyne and Pete Wisdom. What happens to Linda in the House of M is never revealed, but she once again appears as herself following the reversal of the House of M back to normal.
The next time Linda appears, the entire Captain Britain Corps has assembled and are preparing to fight what Roma tells them is a battle to protect all reality. Jim Jaspers and Merlyn have somehow managed to form an alliance and are attempting to regain the control of protecting the Omniverse, a role the mad Merlyn believes his daughter, Roma, has stolen from him. The Corps attacks Jaspers and his army of Furies, and made advances despite losing corpsmen to the Furies. When Jaspers appears to be about to injure Saturnyne – in retaliation for her injuring him – the battle is joined by the Exiles and New Excalibur. Blink, Sabretooth, Thunderbird, and Morph attack Jaspers while Dazzler helps guard Roma from Rouge-Mort.
Somehow, Linda manages to survive the battle, and is still a member of the Corps.
Unlike other Captain Britain Corps members, who are generally assigned to one Earth, Linda has so far been assigned to at least three – Earth-238, Earth-839, and Earth-794 – as well as having operated unofficially on another, Earth-616. It's possible that she acts as a kind of troubleshooter in the Corps, protecting Earths that find themselves without a Corps member of their own for a time.