Forward to the Past by Sloanysoft is a homebrew shooter game made for Sinclair ZX Spectrum.
The story of the game follows Dave and his friend Nate, who work on creating a time machine and travel through portals to the past. The fabric of time is not something to be trifled with, but the time machine gets a little wonky and Dave and Nate get sucked into the time vortex. The only thing now that can save them is collecting 42 MacGuffinanium crystals. This is where you will come in, you will play through all the levels and collect all the crystals required and save the history of time.