In 2001, an unusually straight, narrow ionised filament, 2.5 degrees long, was discovered in the direction of Ursa Major. In 2020, it was recognised as part of a much longer structure – the Ursa Major Arc. The nature of both the filament and the Arc remains unclear, and they may be either galactic or extragalactic. This paper presents the first deep, high-resolution imagery of the filament, revealing structural details along its 10-degree length. It is extremely narrow, with a width ranging from about 70 arcsec to nearly unresolved. It is accompanied by several fainter strands, mostly lying parallel to the main filament.