On a free day, Spike is at Fluttershy helping her out with her animals, when a storm suddenly appears from nowhere. With no other option he stay the night at Fluttershy's. As the night goes on, with the storm still brewing, Fluttershy is awoken by a sudden large clap of thunder. Spike checks on her, and calms her down, and agrees to sleep with her to make her feel better. Where it continues, you'll have to read for yourself.