The allegations are not vague. They allege that representatives of the Reagan presidential campaign made a deal at a meeting in Madrid with Iranians to delay the release of Americans held hostage in Iran until after the November 1980 presidential elections, so that Reagan's opponent, then President Jimmy Carter, whose team had been negotiating, wouldn't gain a popularity boost (an 'October Surprise') before election day. The allegations included a date-specific allegation that William Casey met with an Iranian cleric in Madrid, Spain, and much of the tardy investigations centered on whether, at the weekend in question he was actually at Bohemian Grove retreat in California.
Carter was at the time dealing with the Iranian hostage crisis and the hostile regime of the Ayatollah Khomeini. Those who aver that a deal was made allege that certain Republicans with CIA connections, including George H. W. Bush, arranged to have the hostages held through October, until Reagan could defeat Carter in early November, and then be released. The hostages were in fact released on the day of Reagan's inauguration. If the timing was a double-cross that was meant to tip off the public to the game, it failed to elicit much commentary.
A Public Broadcasting System's 'Frontline' documentary in 1990 brought the story unavoidably to the surface in considerable detail.
Separate House and Senate investigations were further delayed until 1992 however, by which time the trail was safely cold. William Casey, the alleged go-between, was dead by then, and it seemed impossible to account for all his moves during the summer of 1980, when he is said to have conferred with agents representing the Ayatollah Khomeini's government.
If this is how events happened, some believe that dealing with a hostile foreign government to achieve the defeat of a domestic administration would have been an act of treason.
According to Sick's analysis, Oliver North was more or less a fall guy, taking the responsibility in order to conceal the treason of Reagan and Bush.
See also: George Bush family conspiracy theory, Iran-Contra, Iranian hostage crisis