A very odd story. The prefatory section (30 pages) is set in apparently contemporary times, as Craven Derby returns to his childhood home that is now in ruins, and meets an old servant he remembers. The old servant speaks in a strong Worcestershire dialect, and laments the improvements of the age. The old servant has found some old papers hidden away in a secret place, and he passes them on to Craven Derby. They form the basis of the main novel.
The story itself seems a bit like Russian doll - there's the prefatory material which appears broadly contemporary, then it shifts into unspecified bygone days with a group of superstitious servants sitting around a fire telling one another ghost stories. The armourer tells a story of when he was a boy and he saw some supernatural figures (ghosts?) and a father and his son killed by the father's jealous brother. Then 'Peg of the dell' arrives, scares everyone, is given a bed for a night and disappears mysteriously leaving behind a baby of apparently noble birth. The baby grows up to be Ludolph, who falls in love with the daughter of the house, Bona. A visiting Lord named Bellamonte also falls for Bona and tries to kill Ludolph in various ways, but on each occasion Ludolph is rescued by the heavenly presence of the 'Lady of the Rose'. Eventually, Bona's parents recognise Ludoloph's worth and he is promised marriage to Lady Bona, and goes off to fight with King Richard in the crusades. There he impresses King Richard and is elevated to Sir Lionel, Baron of Darbye. He has various encounters (some in disguise) with the Saladine. Saladine is made out to be a very sympathetic character and the novel has a pro-Islam stance. Ludolph's friend Lord Herbert falls in love with the Saladine's daughter and converts to Islam to marry her. At one point they encounter a beautiful Amazon captain, who eventually follows Lionel back to England in disguise.
Meanwhile, back in England Bona visits a nunnery regulary, and hears the story of an elderly nun whose husband and son were killed many years ago. When Ludolph finally returns (just as Bona is about to take the veil herself, believing Ludolph to be dead) it is revealed that Ludolph is the son of the Lord's son from the armourer's ghost story (the boy was not killed but spirited away to France) and therefore the elderly nun's grandson. All ends happily.
Overall, this is a story with way too many ideas - it starts in the present and the character of Frank Feldfalk is developed very sympathetically, then it skips to medieval Britain and the character of the 'Lady of the Rose' is introduced, but actually perhaps the biggest section of the novel is set abroad while Ludolph is off fighting the crusades, and the titular 'Lady of the Rose' makes no appearances at all.
On reading it, what is particularly striking is the apparent contrast between the very 'realistic' and detailed depiction of Frank Feldfair, and the very 'romantic' and unrealistic handling of the main story. The author loves showing off his knowledge and educating the reader, but the main story is very fantastical. For example, the author goes to some length to tell one how long it would have taken in the past to travel from London to Worcester in days gone by, and how people would make wills and bid a final farewell to their families, however in the same story Ludolph sets off on the crusades apparently without a thought that he might not return.

All the dialect is in the preface, where it is lengthy and detailed and creates a very three-dimensional character in the person of the old servant Frank Feldfair. There is just a tiny bit of dialect marking for the housekeeper at the start of the main story, and in fact it is not consistent with Frank's speech despite the fact that they are apparently from the same area, e.g. Frank says 'wool' for 'will' where housekeeper says 'wull'.
The main story makes a lot of use of archaism for the high-born characters, Christian and Muslim alike.
The novel as a whole feels rather antiquarian - there is regular use of footnotes to explain the dialect terms in the preface, and to explain archaic terms in the main text.