When the king of the gods falls head over heels in love with you, seriously bad stuff happens. If you're a Handelian soprano like Semele, however, it happens very tunefully and in a thoroughly pleasant, elegant and cheerful manner for the whole evening.
If you're confused as to the difference between opera and oratorio, Semele is the perfect aid to your confusion. The work has virtually every feature of a baroque Italian opera - a light-hearted love-based plot rather than a worthy biblical one, music that moves the story along and is well suited to staging, a broad cast of characters with the usual operatic balance of voices, etc. Except that it was written to an English libretto by William Congreve, and Handel attempted to disguise it as an oratorio for Lent, presumably because it meant he could get paid. Unsurprisingly, the more religious concertgoers were unimpressed by a tale with such loose morals, and Semele tanked, to languish unheard for nearly 200 years.
Which is a pity, since it's a lot of fun and a great piece of music. It's one of Handel's last operas, and the style is definitely developing: the "da capo" arias which are so damaging to dramatic flow are thin on the ground, the action moves briskly, and there are plenty of good tunes to keep you engaged. There are also a very large number of full-tilt Handel semiquavers to keep the singers on their toes.
All this makes Semele quite a challenging number for a tiny low-budget opera company like Hampstead Garden Opera, but they clearly rose to the challenge with gusto. The production deserves to win prizes in some form of alternative opera awards ceremony: "most creative use of bubble-wrap" for costume designer Rachel Szmukler, "best coloratura performance while being pelted with pillows" for Zachary Devin as Jupiter, "best ability to keep a straight face while wearing a nightie" for Martin Musgrave and Ed Bonner and "best ability to stay still under dust sheets" for too many of the cast to list individually. The production frames the first act as a typical squabbling family - the news from Mount Olympus comes through the ageing TV around which they are clustered - and brings it off entertainingly.