The Slipstream was an experimental aircraft designed and built by Overwatch. It was meant to be a teleporting fighter, but something went catastrophically wrong during its inaugural test flight. The teleportation matrix malfunctioned, and both it and the Slipstream's pilot, Lena Oxton, disappeared.
It was later discovered that Lena was alive, but broken off from the rest of physical reality, resulting in her becoming ghost-like and vanishing from existence for hours or days. Winston, however, developed a harness with teleportation matrix technology that allowed her to become tangible again and ultimately control her new-found time leaping abilities. The Slipstream, however, remains lost.