That is what Data's self-diagnostic was able to determine this morning.
All systems functioning.
Data does a self-diagnostic every morning.
All systems normal.
Now, Data is on the holodeck, enjoying a game of poker with Stephen Hawking, Isaac Newton, and Albert Einstein.
The game is interrupted when the Enterprise goes to red alert. Data reports to the bridge, to find that they are receiving a distress call from Ohniaka III. An unidentified ship is in orbit, and Data is unable to determine if there are life forms present on the surface outpost.
"Worf, Data, you're with me." Commander Riker gets up, and Data follows him and Worf onto the turbolift. They will beam down to the surface to investigate.
All outpost personnel appear to be dead. But electromagnetic interference is making it difficult to determine whether there are any survivors.
Still not particularly strange. They begin a search, and Data goes with Riker to investigate the south wing. That is normal.
The door is jammed. Riker notes that whoever attacked only seemed interested in the people, not the computers. Data bypasses the lock on the door and it opens, revealing a Borg drone on the other side.
The Borg attacks. Riker and Data engage him. More Borg drones appear. They are outnumbered, even when Worf and Corelki come to assist. One of the Borg engages Data hand-to-hand, and Data knocks him to the ground.
Corelki is injured. Another Borg confronts Data, going straight for his neck with his hands. Data retaliates by doing the same, then forcibly removes one of the Borg's hands from his throat, with some effort. Data's other hand clenches…
Something… is not… right.
This is not… normal.
"Stop, stop, stop, STOP!"
Data throws the Borg against the wall. It falls to the ground, barely alive. Data kneels beside the broken body, his hand once again finding the Borg drone's throat. He thrusts its head into the wall and squeezes, until it no longer breathes, until he feels the life drain out of its body. He removes his hand, and he looks at what he has done…
Not normal. Something is… wrong.
Moments later, the drone disappears, along with all of the others. The feeling fades and Data senses Riker and Worf coming up beside him. Data continues to stare at the emptiness where the Borg corpse had been, moments ago. The emptiness is echoed somewhere in his positronic net, making it more obvious what has just been taken away from him, what had sneaked up on him without him realizing it.
Yes, that is what is wrong.
"Data? Data, are you alright?" Commander Riker asks, almost huskily.
"Yes sir," Data answers. Empty.
Data has to think about it. He has no previous record of any such occurrence. Neither can he construct any viable composite from anything in his memory banks. But there is one possibility…
No… that has to be it.
"I got angry," he answers finally, bewildered.
Systems are not functioning within normal parameters.