On February 16, 2006, at 2030 Pacific standard time, a Mooney M20F, N9704M, impacted a tree while on final approach for runway 34 at the Willits Municipal Airport, Willits, California. The airplane was operated by the pilot/owner under the provisions of 14 CFR Part 91 as a personal flight. The private pilot and a passenger sustained minor injuries, while the certified flight instructor (CFI) and another passenger sustained serious injuries. The airplane was substantially damaged. Dark night visual meteorological conditions prevailed, and a flight plan was not filed for the cross-country flight that originated from Sacramento International Airport (SMF), Sacramento, California, at 1846. The flight was destined for Willits. According to the pilot's written statement, he hired the CFI to assist in the night flight, as he was not current in night operations. The pilot and CFI flew from Lampson Field, Lampson, California, to Sacramento to pickup two passengers. The passengers were to be dropped off at Willits and the pilot and CFI were to return to Lampson. The pilot obtained a weather briefing prior to making the uneventful flight to Sacramento. After picking up the passengers the flight proceeded to Willits. After reaching the vicinity of Willits, the pilots activated the pilot controlled lighting over the common traffic advisory frequency. They entered a right downwind for runway 34, which was lit by runway edge lights. As the pilot flew the airplane onto final approach at 80 knots, he entered a low-lying area of ground fog and temporarily lost sight of the runway. The airplane descended too low and the CFI applied full power but the airplane impacted a tree on short final with its right wing. The Airport/Facility Directory (AFD) for Willits indicates that trees were present on the approach end of runway 16 and 34 and medium intensity runway lighting was available. The pilot indicated that some of the runway edge lights were inoperative. The airport was not equipped with a visual approach slope indicator or a precision approach path indicator.