The Sentinels are undone by a design flaw.
The issue follows two basic subplots, following from the previous issue. In the first one, Angel, Cyclops, Iceman and Marvel Girl were captured by the Sentinels, immobilized by a gravitational ray which leaves them barely able to move their limbs. This sub-plot follows their fates as prisoners. The second one has Professor X, stuck outside Sentinel Headquarters return to New York City where a lone Sentinel lies inert in a television studio. He has to discover what knocked out this one and hopefully use it to defeat the rest of them.
The four captive X-Men have been placed in a gravitational globe which keeps them unable to move. This doesn't stop Scott from using his force beams, Jean from using her telekinesis and Robert from using a battering ram made of ice, all in futile attempts to crack open the impenetrable walls of the globe. Angel contemplates that his wings usually make him feel superior to the others, but now seem useless by comparison. (Matching fan letters complaints of the time which often pointed that Warren is the weak link of the X-Men's chain). In a touching scene, Mr. Drake bemoans failing his teammates. Mr. Summers assures him "You did all that any man could do --- you tried your best". Robert is touched because it it is the first time Cyclops acknowledges him as a man, not a mere boy. Besides giving some emotional depth to Iceman, the scene is a rare example of camaraderie involving the emotionally-distant Cyclops.
Back in New York City, Xavier allies himself to the local police force. A quick examination of the fallen Sentinel has him discover what went wrong with it. The Sentinels use an external source of power, their energy transmitted to them from a distance. A giant crystal displayed on a nearby building has interfered with the transmission. Now Xavier figures out how to defeat the rest of the Sentinels.
At the Headquarters, Master Mold has finished mind-probing the Beast. The unconscious prisoner is to be placed with the rest of them. In order for anyone to enter or exit the gravitational globe, the device has to be briefly de-activated. Cyclops had been waiting for exactly such an opportunity and blasts the Sentinel holding the Beast. The other Sentinels present attempt to react, but the poor coordination of their actions has them tripping over each other. Giving enough time for Jean and Warren to exit the cell. Jean waists now time forcing a Sentinel to the ground, while Robert freezes the floor surface to ensure it has trouble getting back to its feet. The escape attempt seems to be going well... before the always reliable Section Leader 1 leads another group of Sentinels in blasting the X-Men from a safe distance.
1 is about to execute the prisoners, when every Sentinel on the base falls inert. The Cavalry has arrived. Actually, its a trio of police helicopters which have transported the giant crystal just above the Sentinels' Headquarters. Xavier has been placed in charge of the police operation, thanks to his contacts in Washington, D.C, and has figured out a way to pull the proverbial plug on every Sentinel. Makes you wonder where their power source was located.
Master Mold and Bolivar Trask are too busy to notice. They have been pre-occupied with creating a device giving birth to new Sentinels, drainng the Headquarter's energy reserves in the process. Master Mold wants an army of numerous Sentinels to help it conquer the planet. Trask has been blackmailed, that if he fails to obey the nearest human settlement will be annihilated. But as eight new Sentinels are created, Trask figures out a way to stop the Master Mold. And it will only cost his life. He smashes a vital machine, causing an explosion which takes out both him and Master Mold.
The X-Men remain the last surviving lifeforms in the base. But the initial explosion has set up a chain reaction. Explosions are occuring left and right, flames circle the X-Men. Iceman can't take the heat and collapses. Beast, who has spend most of the issue sleeping, now has to rescue them. Jean acts fast, levitating Scott to safety. The fires are no safe place for Angel to flap his wings. Jean telekinetically has them work as a glider, transporting Warren and herself to safety. They find an exit door locked. But her telekinesis opens the door mechanism, allowing everyone to escape. She levitates herself out, Warren transports Cyclops, Hank transports Iceman. They barely have time to get to safety before the Headquarters explodes. Stan Lee sums up the moral of the storyline: "Beware the fanatic! Too often his cure is deadlier by far than the evil he denounces".
The issue ends with a cliffhanger. The Xavier Mansion is currently unoccupied. But there is a shadow on its outer walls. Someone has just arrived, someone wearing a helmet. The mysterious stranger is revealed in the next issue to be Magneto, back from outer space.
This is a rather good issue with plenty of good teamwork ans strategic thinking from the X-Men and a touching self-sacrifice from Trask. But the defeat of the Sentinels basically results from a single design flaw, which helps opponents cut them off from their energy supply. An Achilles' heel which has them unable to perform effectively and undermines them as credible threats. Small surprise that the Mark I Sentinels have never really been revived. Subsequent uses are either flashbacks to this storyline or prequels like "Uncanny X-Men #-1" (July, 1997). The Sentinels' concept was however too good to leave behind and Mark II Sentinels would be introduced in "X-Men" vol. 1 #57-59 (June-August, 1969).