Okay, the (terribly longwinded) explanation. If this isn't clear enough, I don't know what is.

Part I – some people have a drug that they want to test on a human. One of them suggests Heero, a fact that should be clear by the time you finish the fic. Heero's the 'someone you can use'; he was a prisoner before the fic started.

Part II – they gave the drug to Heero and it worked perfectly, with Heero asleep and thinking he's in danger and breaking the poor guard's neck. This part's rather easy to understand.

Part III – Shinigami comes to save Heero, but finds him unconscious and won't wake up. The guards tells him Heero has to sleep the drug off or else any outside interference would kill him. Hopefully you've realized, by the end of the fic, that the higher-ups planned for this to happen, so that whoever saved Heero would leave the drug to work its magic in Heero's sleep.

Part IV – in Heero's mind during his dream. In reality Duo's shaking Heero, slapping him, trying to get him to wake up, and he knows something's wrong with Heero so he's terrified and pained since Heero seems to be in lots of pain himself. Heero's mind, though, supplies him with this scenario: he's in OZ clutches, and Duo is, too, and they're using Duo to threaten him, and he's not going to let the OZ people who are shaking him and yelling threats at him win. So he breaks the supposed 'OZ' interrogator's neck, when in actual fact, it's Duo.

Part V – in Heero's mind during his dream. In reality Quatre and Trowa come in and see Duo dead on the floor and Heero still apparently asleep but in 'soldier' mode. Quatre tries to make noise, screaming Heero's name to give a distraction while Trowa sneaks closer. Heero's mind supplies this scenario: there's a gunfight, Duo's hurt so Quatre is screaming, and the footsteps coming closer is the enemy. So he breaks Trowa's neck, too, while Quatre screams 'No!'

Part VI – in Heero's mind during his dream. In reality Wufei arrives and sees Duo and Trowa's bodies, and is horrified. Quatre can't explain well, too much in shock and grief, only managing to say it's Heero and that he's hiding behind – er, something. In Heero's mind, though, the gunfight continues, and Quatre is breaking down because something bad happened to Duo and Trowa, but he's more worried about Duo.

Then his mind supplies the sound of footsteps, and thinks it's enemy reinforcements. He hears Wufei telling Quatre to 'stay back', so he thinks Wufei went out himself – when actually Wufei's trying to get close to Heero. Then Heero senses a presence and jumps, breaking Wufei's neck as Quatre finally intervenes, trying to help Wufei but it's too late, and he gets his own neck broken.

Part VII – Heero wakes up, throat screamed raw and memories/fragments from his dream flashing through his mind, and his friends' and his own yells echoing in his ears. Then his eyes clear, and he looks around, and he screams.