UNIT continued to feature in "Doctor Who" after "Spearhead", but when the Third Doctor's exile is lifted in "The Three Doctors" (1972–73), his association with UNIT becomes more sporadic, especially after his regeneration into his fourth incarnation at the end of "Planet of the Spiders" (1974). The last appearance of UNIT in the series for many years was in "The Seeds of Doom" (1976); however, the organisation continues to execute its mandate to investigate and combat alien activity. The final appearance of UNIT during the original run of "Doctor Who" was the Seventh Doctor serial "Battlefield" (1989).