After Naked Snake/Big Boss is tortured, loses his eye, and escapes from a fortress, he meets up with a female spy named EVA. While he's recovering and eating around a fire, the two see a butterfly fluttering around. Snake looks up at it and decides to test his now-impaired depth perception by grabbing at it. He misses. Depressed, he slowly feels his new eyepatch, while EVA tries to cheer him up.
The next morning, she gives Snake some C3 (a predecessor to C4) so that he can blow up the hangar that the Shagohod is stored in. As a cute token, she molds a piece of C3 into a heart.
Later, when you plant the final charge of C3, it'll show a cutscene of Snake stopping for a second to look at the heart-shaped C3 instead of sticking it onto the detonator. He molds it into the shape of a butterfly, like the one that he couldn't grab. He throws it up into the air and dramatically catches it and slaps it onto the detonator.
"Gotcha this time."
http://youtube.com/watch?v=C3bGN46NE_o
