You and Nick have known each other for a bit and were traveling on a bridge together. The two of you were on your way to take care of some raiders that have been a pain in the ass for Diamond City. While the two of you were on the bridge that crossed a river, y'all had a run in with a group of raiders...

You and Nick fought against the raiders but you were knocked into the river below... Nick being a synth, he couldn't go into large bodies of water for obvious reasons. But, he couldn't loose you... The water was irradiated of course and who knew what mutated creatures were in the depths of that river...