Imagine if you will a vast generation-ship sent on a mission of colonization; a ship whose fifteen-year trip is unforeseeably prolonged to a thousand, its passengers living and dying (like, as it were, mayflies) through successive generations ... Good grief, you're asking, "who hasn't written this one?"
Weirdly, O'Donnell makes it work. The mainspring is the ship's computer, programmed to fail safe in every way – no nonsense about its breaking down and leaving the passengers to revert to barbarism. Rigid programming, of course, brings its own problems. For reasons not clearly stated (nor over-convincing in the light of microelectronic progress) the computer incorporates a human brain. There is a ghost in the machine, the traumatized proprietor of said tissue, who wakes up as the ship departs, threshes around mentally until he turns off the drive fourteen years and eleven months too soon, and spends much of the book coming to terms with (a) not having a body; (b) having a body after all, i.e. the ship; (c) an artificial alter ego, "The Program", which in its fail-safe way can override him on every front but isn't flexible enough to cope with the problems of the extended trip. The three-way conflict between this impersonal Program, the impotent hero and the uncomprehending latter-generation colonists is interesting and well told.
Like a vastly less polished Michael Bishop, O'Donnell deploys hordes of vividly-realized people and somehow gets away with many a ream of unlikely future slang. (Rather than kill people you "term" them, this being a significant advance on O'Donnell's last book Bander Snatch in which you could only "cough" them: ugh.) There is a welcome terseness; important plot turns may whiz past almost too swiftly for comfort, but they are not dwelt upon and hammered home and restated in numerous ways for the benefit of a presumed moronic audience.
The trouble with terseness is that when incidents don't stretch too far, one must introduce more and more incidents. For the most part this works: there's plenty of well-handled aberrant behaviour among the colonists and moral strife in the computer/hero. But the temptation to introduce vast numbers of arbitrary events becomes too great, and deus after deus ex machina pops up. Earth develops FTL travel and zooms ahead of the colony ship; nasty aliens "rape" the ship; nice alien visits; nasty aliens return; some of this, however neatly tied in, is surely being inserted to make the book longer. True, the evil aliens' computer conveniently teaches Our Hero how to beat the Program (why? Oh, it just felt like it); but he was already making slow, convincing progress as the reluctant Program was forced to hand over partial control in sector after sector because of its inability to deal with unpredicted situations.
Mind-to-mind strife in computer space can be dull indeed, in a grey fog of power thrusts and impalpable thought screens. O'Donnell realizes that a wired-up brain will see inputs as solid metaphors – though metaphors subject to reinterpretation. The undersea imagery in scenes like the death struggle between ego and Program is good, showing the teeth of the threats (electronic "virus" programs aimed to erase our hero's identity are seen as torpedoes which must be individually intercepted, etc) while keeping the dreamlike quality proper to a conflict with no moving parts. Less solemn metaphors also appear. The ship's takeoff mocks yet reinforces the wonder of spaceflight as our hero awakens to an interpretation of his jet-propelled status ... farting his way to the stars.
The trouble with an immortal hero/computer is that everyone else flits by too swiftly, however well handled in their brief mayfly moment on the stage. And the ending is pure wish-fulfilment, with everybody getting all they wish for. And the prose slithers into journalese when O'Donnell's concentration slips, or sometimes into bathos when he's trying especially hard. And there are scientific glitches: "radio waves on a frequency inaudible to humans", indeed! The book is still enjoyable, with an inventiveness, ambition and ability to write which rarely meet.in one author these days. For all its rough edges, Mayflies is recommended to those who wondered what had happened to hard sf.