  We present new neutral hydrogen (HI) observations of the nearby galaxy NGC
2403 to determine the nature of a low-column density cloud that was detected
earlier by the Green Bank Telescope. We find that this cloud is the tip of a
complex of filaments of extraplanar gas that is coincident with the thin disk.
The total HI mass of the complex is $2\times10^{7}\text{ M}_\odot$ or 0.6% of
the total HI mass of the galaxy. The main structure, previously referred to as
the 8-kpc filament, is now seen to be even more extended, along a 20 kpc
stream. The kinematics and morphological properties of the filaments are
unlikely to be the result of outflows related to galactic fountains. It is more
likely that the 20 kpc filament is related to a recent galaxy interaction. In
this context, a $\sim$ 50 kpc long stellar stream has been recently detected
connecting NGC 2403 with the nearby dwarf satellite DDO 44. Intriguingly, the
southern tip of this stream overlaps with that of 20 kpc HI filament. We
conclude that the HII anomalies in NGC 2403 are the result of a recent
($\sim2\text{ Gyr}$) interaction with DDO 44 leading to the observed
filamentary complex.
