On August 19, 2003, at 0001 central daylight time, a Beech BE-76, N1812S, sustained substantial damage during a departure from controlled flight and subsequent recovery near Hammond, Minnesota. The private pilot and two passengers reported no injuries. Instrument meteorological conditions (IMC) prevailed at the time of the accident. The personal flight was operating under 14 CFR Part 91 on an instrument flight rules (IFR) flight plan. The flight departed Watertown Municipal Airport (ATY), Watertown, South Dakota, on August 18, 2003, at 2216, with Quad City Airport (MLI), Moline, Illinois, as the destination airport. According to the pilot's written statement, he did not fly a direct route to MLI due to thunderstorms that extended from northeast Nebraska to southwest Minnesota. The pilot stated his route of flight was ATY to St. Cloud, Minnesota, and then to Minneapolis, Minnesota. He reported the route of flight was intended to circumnavigate the line of thunderstorms and to remain in visual meteorological conditions (VMC). The pilot stated that the flight was at 7,000 feet mean seal level (msl) on a 130 magnetic heading when the airplane entered a cloud and experienced IMC conditions and light turbulence. The pilot reported that he disengaged the autopilot subsequent to entering IMC. The pilot stated he "had been hand flying the airplane for several minutes when we hit a couple of waves of heavier turbulence followed by nearby lightning flash." The pilot reported the heavier turbulence resulted in an altitude loss of approximately 100 feet. The pilot stated he attempted a minor control input to correct for the loss of altitude and the airplane departed from controlled flight. The pilot reported, "The airspeed indicator went full scale deflection up, despite retarding throttles, and [the] VSI was full scale deflection down. I felt that we were in a spiral/spin and I attempted to reverse this, but also felt largely unsuccessful in my efforts. We exited the clouds in a vertical nose down orientation without rotation/spin. An aggressive control input was made to arrest the descent and then a climb was started." The pilot regained control of the airplane after losing approximately 4,000 feet. He reported that he was unaware of what actions he took to recover from the spin, but he was able to return to level flight and continue on to his destination at the assigned altitude. He made an uneventful landing at MLI. When he departed the airplane he discovered substantial "wrinkled" damage to the airplane's wings.